Depends where the movie was filmed and at what time from what I've seen. Movies like Rambo III had access to real Soviet weaponry and a real DShK is used in that, a real one is also used in Shooter and the Wolverine movie. (only ones that come to mind currently) but in the US I assume armorers have a lot of those mocked up DShKs and .50 BMG blanks are easier to aquire than 12.7x109mm rounds. But if authenticity is requested I assume they'll use real ones at this point.
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